Shape memory materials are those materials that can be “fixed” to a temporary and dormant shape under specific conditions of temperature and stress and later, under thermal, electrical, or environmental command, the associated elastic deformation can be completely or substantially relaxed to the original, stress-free, condition.
One class of shape memory materials studied and utilized are the shape memory alloys (SMA). The shape memory capabilities of the various metallic materials (shape memory alloys) capable of exhibiting shape memory characteristics occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. In particular, alloys of nickel and titanium, for example, nitanol exhibit these properties of being able to undergo energetic crystalline phase changes at ambient temperatures, thus giving them shape memory. Such alloys have shape memory effects that exploit the deformation behavior difference between a high temperature austenite phase (parent phase) and the room temperature martensite phase, a first-order phase transition separating the two phases. With a temperature change of as little as about 10° C., this alloy can exert a stress as large as 415 MPa when applied against a resistance to changing its shape from its deformed state. Such alloys have been used for such applications as intelligent materials and biomedical devices. Their applications, however, have been limited in part because they are relatively expensive, but also due to limited strain, ca 8%.
Shape memory polymers (SMPs) are being developed to replace or augment the use of shape memory metal alloys (SMAs), in part because the polymers are light in weight, high in shape recovery ability, easy to manipulate, and because they are economical as compared with SMAs.
Polymers intrinsically show shape memory effects on the basis of rubber elasticity but with varied characteristics of temporary shape fixing, strain recovery rate, work capability during recovery, and retracted state stability. The first shape memory polymer (SMP) reported as such was cross-linked polyethylene; however, the mechanism of strain recovery for this material was immediately found to be far different from that of the shape memory alloys. Indeed, a shape memory polymer is actually a super-elastic rubber. When the polymer is heated to a rubbery state, it can be deformed under resistance of ˜1 MPa modulus. When the temperature is decreased below either a crystallization temperature (Tm) or glass transition temperature (Tg), the deformed shape is fixed by the higher rigidity of the material at lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. When the temperature is raised above transition temperature (Tg or Tm), the polymer will recover to its original form as driven by the restoration of network chain conformation entropy. Thus, favorable properties for SMPs will closely link to the network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs have an advantage of high strain (to several hundred percent) because of the large rubbery compliance while the maximum strain of the SMA is less than 8%. As an additional advantage, due to the versatility of polymer, the properties of SMPs can be tailored according to the application requirements, a factor that is very important in industry.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress, particularly at the melting point or glass transition temperature of the soft segment. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above its melting point or glass transition temperature. Also, the hardness of the material changes dramatically when it is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles.
Heretofore, numerous polymers have been found to have particularly attractive shape memory effect, most notably the polyurethanes, polynorbornene, styrene-butadiene copolymers, and cross-linked polyethylene. Other SMPs include alkyated, cyano, alkoxylated mono or diesterified imides or carboxylic acid derivatives. In addition, copolymers and homopolymers of dimethaneoctahydronapthalene (DMON) are known. However, the processing of these polymers has given rise to numerous difficulties.
In the literature, polyurethane-type SMPs have generally been characterized as phase segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition of the hard segment.
Examples of polymers used to prepare hard and soft segments of known SMPs include various polyacrylates, polyamides, polysiloxanes, polyurethanes, polyethers, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; U.S. Pat. No. 5,665,822 to Bitler et al.; and Gorden, “Applications of Shape Memory Polyurethanes,” Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, SMST International Committee, pp. 115-119 (1994).
It has also been proposed to use highly crosslinked homopolymers with Tg>room temperature and long-lived entanglements serving as crosslinks. However, the use of entanglements as the sole origin of elasticity leads to significant difficulties in the processing thus leading to the required use of plasticizers that ultimately hamper shape memory performance. Existing shape memory polymers have been prepared on the basis of polyurethane (Mitsubishi), and Norsorex™ (Nippon Zeon) and used as a rubber. Neither can be cast to complex shape without the use of solvents. The aforementioned severe limitations emphasize the need for castable, reactive formulations, in which the stress-free state is formed during the polymerization process itself. In such a case, shape memory castings (solid objects), films, coatings, and adhesives could all be processed from the same formulation but altered processing schemes.
Recently, a process to make molds for casting composite parts out of novel shape memory polymers has been described in U.S. patent application Ser. No. 10/056,182 of common assignment herewith. In that invention, a liquid resin is injected into a mold thermoformed using a SMP sheet, and after the part cures, the mold is simply raised above the Tg of the SMP, which allows the mold to retract to a flat sheet. Molds made of this class of polymer that possess shape memory mechanical properties have the greatest advantage over conventional metal molds in the demolding. Molds composed of these SMPs create a gentle, automated, and simple demolding process. Conventional, rigid material molds have a tendency to remain adhered to the parts in high detailed areas; therefore, most demolding actions are usually very violent. This means that a large percentage of parts that were of good quality before the demolding process are unusable after this step in conventional processes. The slow natural retracting motion of the SMP class of polymers helps to solve this problem. Also, since the SMP material returns to the original shape given, it can be ready formed into another mold. By utilizing the properties that the SMPs possess, many problems that are found in current metal molds can be solved. The SMP material can be easily produced and is inexpensive. The SMP can be made to retain intricate detail. Also, another favorable property of the SMP is that it is transparent. This allows visual inspection of the mold and article therein during the injection and cure steps.
Composite parts have previously been cast in molds made of thermoplastics. However, the use of shape memory polymers as molds was not previously feasible in industrial applications. This was due to the fact that the glass transition temperatures of the SMP materials available were too low. Therefore in most cases, the temperature that was required to cure the resin to form the desired part was higher than the glass transition temperature of the SMP material. This meant that the mold itself would deform back to its original shape before the resin part had a chance to cure.